1. Field of the Invention
This invention relates to the production of ozone for use in a variety of processes such as decontamination of water. More specifically, the invention relates to an electrochemical cell and a process for generating ozone in the electrochemical cell.
2. Background of the Related Art
Ozone has long been recognized as a useful chemical commodity valued particularly for its outstanding oxidative activity. Because of this activity, it finds wide application in disinfection processes. In fact, it kills bacteria more rapidly than chlorine, it decomposes organic molecules, and removes coloration in aqueous systems. Ozonation removes cyanides, phenols, iron, manganese, and detergents. It controls slime formation in aqueous systems, yet maintains a high oxygen content in the system. Unlike chlorination, which may leave undesirable chlorinated organic residues in organic containing systems, ozonation leaves fewer potentially harmful residues. Ozone has also been shown to be useful in both gas and aqueous phase oxidation reactions which may be carried out by advanced oxidation processes (AOPs) in which the formation of OH.radicals is enhanced by exposure to ultraviolet light. Certain AOPs may even involve a catalyst surface, such as a porous titanium dioxide photocatalyst, that further enhances the oxidation reaction. There is even evidence that ozone will destroy viruses. Consequently, it is used for sterilization in the brewing industry and for odor control in sewage treatment and manufacturing. Ozone may also be employed as a raw material in the manufacture of certain organic compounds, e.g., oleic acid and peroxyacetic acid.
Thus, ozone has widespread application in many diverse activities, and its use would undoubtedly expand if its cost of production could be reduced. In addition, since ozone is explosive when concentrated as either a gas or liquid, or when dissolved into solvents or absorbed into cells, its transportation is potentially hazardous. Therefore, its is generally manufactured on the site where it is used. However, the cost of generating equipment, and poor energy efficiency of production has deterred its use in many applications and in many locations.
On a commercial basis, ozone is currently produced by the silent electric discharge process, otherwise known as corona discharge, wherein air or oxygen is passed through an intense, high frequency alternating current electric field. The corona discharge process forms ozone through the following reaction: EQU 3/2O.sub.2 =O.sub.3 ; .DELTA.H.degree..sub.298 =34.1 kcal
Yields in the corona discharge process generally are in the vicinity of 2% ozone, i.e., the exit gas may be about 2% O.sub.3 by weight. Such O.sub.3 concentrations, while quite poor, in an absolute sense, are still sufficiently high to furnish usable quantities of O.sub.3 for the indicated commercial purposes. Another disadvantage of the corona process is the production of harmful NO.sub.x otherwise known as nitrogen oxides. Other than the aforementioned electric discharge process, there is no other commercially exploited process for producing large quantities of O.sub.3.
However O.sub.3 may also be produced by the electrolytic process, wherein an electric current (normally D.C.) is impressed across electrodes immersed in an electrolyte, i.e., electrically conducting, fluid. The electrolyte includes water, which, in the process dissociates into its respective elemental species, O.sub.2 and H.sub.2. Under the proper conditions, the oxygen is also evolved as the O.sub.3 species. The evolution of O.sub.3 may be represented as: EQU 3H.sub.2 O=O.sub.3 +3H.sub.2 ; .DELTA.H.degree..sub.298 =207.5 kcal
It will be noted that the .DELTA.H.degree. in the electrolytic process is many times greater than that for the electric discharge process. Thus, the electrolytic process appears to be at about a six-fold disadvantage.
More specifically, to compete on an energy cost basis with the electric discharge method, an electrolytic process must yield at least a six-fold increase in ozone. Heretofore, the necessary high yields have not been realized in any forseeably practical electrolytic system.
The evolution of O.sub.3 by electrolysis of various electrolytes has been known for well over 100 years. High yields up to 35% current efficiency have been noted in the literature. Current efficiency is a measure of ozone production relative to oxygen production for given inputs of electrical current, i.e., 35% current efficiency means that under the conditions stated, the O.sub.2 /O.sub.3 gases evolved at the anode are comprised of 35% O.sub.3 by volume. However, such yields could only be achieved utilizing very low electrolyte temperatures, e.g., in the range from about -30.degree. C. to about -65.degree.. Maintaining the necessary low temperatures, obviously requires costly refrigeration equipment as well as the attendant additional energy cost of operation.
Ozone, O.sub.3, is present in large quantities in the upper atmosphere in the earth to protect the earth from the suns harmful ultraviolet rays. In addition, ozone has been used in various chemical processes, is known to be a strong oxidant, having an oxidation potential of 2.07 volts. This potential makes it the fourth strongest oxidizing chemical known.
Because ozone has such a strong oxidation potential, it has a very short half-life. For example, ozone which has been solubilized in waste water may decompose in a matter of 20 minutes. Ozone can decompose into secondary oxidants such as highly reactive hydroxyl (OH.) and peroxyl (HO.sub.2.) radicals. These radicals are among the most reactive oxidizing species known. They undergo fast, non-selective, free radical reactions with dissolved compounds. Hydroxyl radicals have an oxidation potential of 2.8 volts (V), which is higher than most chemical oxidizing species including O.sub.3. Most of the OH.radicals are produced in chain reactions where OH.itself or HO.sub.2.act as initiators.
Hydroxyl radicals act on organic contaminants either by hydrogen abstraction or by hydrogen addition to a double bond, the resulting radicals disproportionate or combine with each other forming many types of intermediates which react further to produce peroxides, aldehydes and hydrogen peroxide.
Electrochemical cells in which a chemical reaction is forced by added electrical energy are called electrolytic cells. Central to the operation of any cell is the occurrence of oxidation and reduction reactions which produce or consume electrons. These reactions take place at electrode/solution interfaces, where the electrodes must be good electronic conductors. In operation, a cell is connected to an external load or to an external voltage source, and electric charge is transferred by electrons between the anode and the cathode through the external circuit. To complete the electric circuit through the cell, an additional mechanism must exist for internal charge transfer. This is provided by one or more electrolytes, which support charge transfer by ionic conduction. Electrolytes must be poor electronic conductors to prevent internal short circuiting of the cell.
The simplest electrochemical cell consists of at least two electrodes and one or more electrolytes. The electrode at which the electron producing oxidation reaction occurs is the anode. The electrode at which an electron consuming reduction reaction occurs is called the cathode. The direction of the electron flow in the external circuit is always from anode to cathode.
Recent ozone research has been focused primarily on methods of using ozone, as discussed above, or methods of increasing the efficiency of ozone generation. For example, research in the electrochemical production of ozone has resulted in improved catalysts, membrane and electrode assemblies, flowfields and bipolar plates and the like. These efforts have been instrumental in making the electrochemical production of ozone a reliable and economical technology that is ready to be taken out of the laboratory and placed into commercial applications.
However, because ozone gas has a very short life, it is preferably generated in close proximity to where the ozone will be consumed and at a rate substantially equal to the rate of consumption. Because so many of the present applications for ozone deal with the oxidation of contaminants in water streams, air streams and soil, it is typically impractical to bring the contaminant to a centralized ozone processing plant. Rather, it is imperative that the ozone be generated at the site of the contamination. This may be an active or abandoned industrial site or a remote location where little or no utilities are available. Furthermore, the rate of ozone consumption will vary according to the type of decontamination process and the nature of the site itself.
Unfortunately, there has been very little attention given to the development of self-contained and self-controlled support systems and utilities for ozone producing electrochemical cells. In order for these systems to be commercially successful, the systems must be reliable, require low maintenance, operate efficiently and be able to operate on standard utilities, such as 110 V, 60 Hz AC electricity provided by a standard gasoline powered generator. Furthermore, these objectives must be met while providing a simple system that can be used to decontaminate a site in a cost-effective manner.
Therefore, there is a need for an ozone generator system that operates efficiently on standard AC electricity and water to deliver a steady and reliable stream of ozone gas. It would be desirable if the system was self-contained, self-controlled and required very little maintenance. It would be further desirable if the system could provide a continuous supply of ozone at a rate dependent upon demand.